A significant challenge that communications equipment designers face is how to pack more information into a given amount of radio frequency spectrum or bandwidth. Frequency division multiplexing (FDM) is a known technique for packing the transmission of data or information into several closely-spaced channels or subcarriers within a predetermined signal bandwidth. FDM systems may separate subcarrier frequency spectra by using frequency guard bands to avoid interference among the subcarriers. Unfortunately, this interference avoidance technique may increase system overhead and degrade bandwidth efficiency.
Orthogonal frequency division multiplexing (OFDM) provides a more robust technique for efficiently transmitting data using several subcarriers within a prescribed channel bandwidth. The OFDM method arranges the subcarriers for greater efficiency as compared with the FDM method that employs guard bands. More particularly, OFDM overlaps the spectra of the OFDM subcarriers to more efficiently pack the subcarriers into the available channel bandwidth. However, to avoid interference among the OFDM subcarriers, the OFDM technique typically requires that the subcarriers remain orthogonal to one another.
Many contemporary cellular communication systems employ OFDM technology as a way to embed information on a radio frequency signal. Cellular systems typically divide up a desired radio coverage area into a number of smaller geographic areas referred to as cells. Each cell includes a base station generally located at or near the center of the cell. The system assigns different radio frequencies to base stations in adjacent cells to avoid interference between adjacent cells. Mobile station users communicate with other mobile station users in the same or other cells via radio OFDM links through the base stations.
Cellular systems that employ OFDM may broadcast the same information simultaneously from all the cells of the system or from a subset of the cells. The cells or subset of cells form a broadcast zone. A mobile station receiver in the broadcast zone may potentially receive signal from all cells in the broadcast zone. A single frequency network (SFN) may be formed by synchronizing all the cells in the broadcast zone and employing OFDM as the communication mode. In such an SFN system, the signal to interference plus noise ratio (SINR) may be improved because a mobile station's receiver may collect the signal from all the cells in the broadcast zone without interference except for background noise and signals from other broadcast zones. This SFN OFDM technique may thus achieve improved recovery of broadcast information in comparison to other systems.
In conventional SFN OFDM systems, each base station in the broadcast zone may transmit a single stream of broadcast traffic. Unfortunately, the increase in broadcast traffic capacity with the signal-to-interference-plus-noise ratio (SINR) is logarithmic. Thus, for larger SINR, doubling of SINR results in a relatively low increase in broadcast traffic capacity. Although reception is improved, this method results in an inefficient use of valuable radio frequency spectrum.
What is needed is a wireless communication system that addresses the bandwidth efficiency problems discussed above.